In the early stages of the primary, most Democratic presidential candidates have avoided discussing Special Counsel Robert Mueller's investigation into 2016 Russian election interference and the Trump campaign's conduct surrounding the meddling, instead focusing on policy. But after Attorney General William Barr released the redacted version of Mueller's report last Thursday, it has started come to the forefront, and talks of impeachment have followed.

Sen. Kamala Harris (D-Calif.) on Monday evening became the fourth candidate to officially back Congress beginning impeachment proceedings against President Trump during a CNN town hall at Harvard University.

“I believe Congress should take the steps towards impeachment.”

Sen. Kamala Harris says the Mueller report points toward obstruction, but adds she is a “realist” and doubts her Republican colleagues in the Senate will vote to remove President Trump from office. #HarrisTownhallpic.twitter.com/k1B154bzql

Harris' announcement follows Sen. Elizabeth Warren (D-Mass.), who was the first candidate to do so last Friday. Former Housing and Urban Development Secretary Julián Castro and Miramar, Florida, Mayor Wayne Messam (D) also said they believed the Mueller report reveals grounds for impeachment, per The Hill. South Bend, Indiana Mayor Pete Buttigieg said that Trump "deserves" impeachment, but the issue should be left to Congress.

Most Democrats in the ever-growing field, including frontrunners like Sen. Bernie Sanders (I-Vt.), have argued that Congress should continue investigating Trump before deciding whether to launch the impeachment process. Tim O'Donnell

Sen. Elizabeth Warren (D-Mass.) on Friday called on the House to initiate impeachment proceedings against President Trump. Warren said her reasoning is based on the findings of Special Counsel Robert Mueller, which Attorney General William Barr made available — with redactions — to Congress and the public on Thursday.

The severity of this misconduct demands that elected officials in both parties set aside political considerations and do their constitutional duty. That means the House should initiate impeachment proceedings against the President of the United States.

In an appearance on CNN's The Rachel Maddow Show on Friday evening, Warren added that "the report is absolutely clear that a foreign government attacked our electoral system to help Donald Trump."

This is a good answer from Elizabeth Warren on why Democrats should at least initiate an impeachment inquiry. It’s not about politics; it’s about what presidents can be allowed to do, and letting him get away with this is a terrible precedent. pic.twitter.com/aONjC57pYu

From now until New Year's Eve, for 10 minutes every hour, ads calling for the impeachment of President Trump will greet the masses moving through Times Square in the heart of Manhattan.

The digital billboards are paid for by billionaire environmental activist Tom Steyer's political group Need to Impeach. Steyer will spend $20 million on the billboards and television ads, which ask citizens to sign a petition urging elected officials to impeach Trump. "We legitimately feel that this is the huge issue in front of the American people and that no one is standing up for what the overwhelming number of Americans think," Steyer told Bloomberg Politics on Monday.

The billboards went up Monday, and the commercials started airing in October. In the ads, Steyer says Trump is a "clear and present danger" to the United States, helping move the country closer to nuclear war. Catherine Garcia

Thousands of people rallied in several cities across the country on Sunday to call on Congress to impeach President Trump. In Los Angeles, several thousand people marched carrying signs saying "Worst President Ever" and "We demand the truth," and chanting, "Down, down, down with Trump — up, up, up with the people." "Every day when I wake up, something is more terrible than it was yesterday," said protester John Meranda, 56, of Long Beach. In many of the cities, the demonstrations were met with counter-protests by Trump supporters. "There's no reason at this point to impeach the president. He's committed no crimes. There's no reason," Jake Lloyd, who backs the president, said in Austin, Texas. "I don't think they understand what impeachment is." Harold Maass

In the wake of last night's House vote to sue the president, new data from Nate Silver at FiveThirtyEight suggests that Democrats are actually talking about the possibility of impeachment of President Obama more than Republicans. Eleven out of 14 mentions of impeachment on the congressional record this July come from Democratic members of Congress; and the left-leaning MSNBC mentions impeachment five times more often than the more conservative Fox News.

Meanwhile, Democrats have raised more than $2 million via emails talking up the threat of impeachment hearings. The emails — sent in the names of VP Joe Biden, Rep. Nancy Pelosi, and other high-ranking Democrats — frequently imply that impeachment proceedings could begin at any moment, featuring subject lines like "Impeachment (TODAY)."

Silver notes that in 2006, Republicans similarly tried to rally their base against the possibility of impeachment. At that time, Fox mentioned the topic almost twice as often as MSNBC. For more information on the history of calls for Obama's impeachment, check out this timeline, which tracks the topic's development since 2009. Bonnie Kristian

House Majority Whip Steve Scalise (R-La.) on Sunday repeatedly declined to give a straight answer on whether he would support impeaching President Obama.

Even in the relatively friendly confines of Fox News Sunday, Scalise deflected three times when asked by host Chris Wallace whether impeachment was on the table. The first two times, Scalise said the White House was pushing the impeachment narrative to boost fundraising and rally its base, though he declined to say whether the GOP would actually pursue impeachment as some of its most conservative members have urged.

When Wallace tried for a third time to elicit a direct answer, Scalise changed the subject once more, responding, "The White House will do anything they can to change the topic away from the president's failed agenda." --Jon Terbush

Former Vice President Dick Cheney appeared Tuesday on CNN, where he slammed President Obama and alleged that the president does not even believe in having a strong national security policy for the United States. Cheney does not, however, favor impeaching Obama, as other conservatives like Sarah Palin are now agitating to do:

I'm not prepared at this point to call for the impeachment of the president. I think he is the worst president of my lifetime; I fundamentally disagree with him, and I think he's doing a lot of things wrong.... But I think that gets to be a bit of a distraction — just like the impeachment of Bill Clinton did. Everybody could get geared up to have a big fight over it, but it wasn't going anyplace. [Cheney via CNN]

The discussion about impeachment begins at just after the 3:50 mark in the video below. --Eric Kleefeld